A variety of filters have found use in many fields. They include filters to remove trace gas from a clean room, filters for air cleaning, air conditioning, and ventilation, and filters that admit outside air into the automotive cabin (cabin filter). Filters are used also in fuel cells which are attracting attention recently. They are required to remove impurities from air (as an oxygen source) being supplied to the fuel cell. Thus, applications of filters are expanding more and more.
Under these circumstances, an ability to both collect dust and remove gaseous components is required of filters for air cleaners, air conditioners, ventilators, and automotive air ducts to admit outside air into the cabin.
Meanwhile, fuel cells are expected to be a promising technology to cope with global warming. For their operation, fuel cells need air as an oxygen source to be supplied to the oxygen electrode. Unfortunately, air contains sulfur compounds and organic compounds detrimental to the catalytic action in fuel cells. For stable output over a long period of time, fuel cells should be supplied with as pure air as possible after removal of impurities from outside air. This calls for filters with high deodorizing (gas removing) performance and high dust collecting performance. Moreover, filters for fuel cells should have low pressure loss as well as high deodorizing and dust collecting performance because auxiliary equipment (such as blowers) should run with low energy consumption for improved energy conversion efficiency.
It has been common practice to collect comparatively fine dust smaller than 10 μm (referred to as fine dust hereinafter) by using filters made of polypropylene fiber, polyester fiber, polyamide fiber, glass fiber, or the like. Such filters are nonwoven fabrics produced from polymers by spun bonding or melt blowing. There is known a charged filter which is highly capable of collecting fine dust by Coulomb force due to charge on an easily chargeable non-polar polymer such as polypropylene, as disclosed in Japanese Patent Publication Nos. Sho 56-47299, Sho 57-14467, and Hei 5-15485, and Japanese Patent Laid-open Nos. Sho 61-46215, Hei 4-213335, and Hei 7-227510.
There are other filters such as net-like ones formed from polypropylene or polyethylene resin and those of three-dimensionally reticulated polyurethane foam, which are used for collection of comparatively coarse dust larger than 10 μm (referred to as coarse dust hereinafter).
Unfortunately, the former fibrous filters are liable to clogging, which decreases efficiency soon, despite their good ability to collect fine dust, and the latter net-like filters lack an ability to collect fine dust.
There has been proposed a filter to achieve dust removal and deodorization simultaneously which is formed into a desired shape from a deodorizing agent and a dust removing material by using a binder (Japanese Patent Laid-open No. Hei 8-117524). This filter, however, is not necessarily good in deodorizing performance because it merely permits a limited amount of deodorizing agent to be incorporated thereinto so that it keeps its low initial pressure loss. Another filter for simultaneous deodorization and dust removal is one which is a simple laminate composed of a deodorizing layer and a dust collecting layer. It has the disadvantage of being poor in deodorizing and dust removing performance per unit amount of pressure loss.
There has been proposed another type of filter which is composed of two layers of nonwoven fabrics (both capable of dust collection) laminated on top of the other, with deodorizing particles attached to the inside of one layer by means of a hot-melt adhesive. Unfortunately, this filter is inherently liable to clogging if it is intended mainly to collect fine dust, and this prevents incorporation with a sufficient amount of deodorizing agent. Conversely, the filter will be very poor in ability to collect fine dust if it is sufficiently incorporated with a deodorizing agent.
As explained above, conventional filters cannot exhibit both dust collecting ability and deodorizing ability when they are used in areas, such as ventilation, air conditioning, automobiles, and fuel cells, where an increase in initial pressure loss after filter mounting or an increase in pressure loss due to clogging with dust is not permissible because it lowers the blowing capacity. They do not meet all the requirements.
There has been proposed a pleated filter member which is expected to have both dust collecting ability and deodorizing ability. It is made of polyurethane foam with three-dimensionally reticulated skeletons free of cell membrane, and it is used as the base of deodorizing filters or dust collecting filters. Although it collects coarse dust efficiently without becoming clogged easily and it is also capable of deodorization, it is limited in the amount of deodorizing agent to be incorporated thereinto because it increases in initial pressure loss if it is designed for efficient collection of fine dust.